The present invention relates to a plasma arc torch and, more particularly, to a novel electrode for use in a plasma arc torch having an improved service life and a method of making the same.
Commonly used for working of metals, plasma arc torches are used for cutting, welding, surface treatment, melting and annealing. These torches include an electrode that supports an arc that extends from the electrode to the workpiece in the transferred arc mode of operation. It is also conventional to surround the arc with a swirling vortex of gas, and in some torch designs, it is conventional to envelope the gas and arc with a swirling jet of water.
The electrode used in a conventional torch of the type described typically comprises an elongate tubular member composed of a material of high thermal conductivity, such as copper or copper alloy. The forward or discharge end of the tubular electrode includes a bottom end wall having an emissive insert embedded therein, which supports the arc. The insert is composed of a material which has a relatively low work function, which is defined in the art as the potential step, measured in electron volts, which permits thermionic emission from the surface of a metal at a given temperature. In view of its low work function, the insert is thus capable of readily emitting electrons when an electrical potential is applied thereto, and commonly used insert materials include hafnium, zirconium, and tungsten.
One of the major problems connected with the torches referred to above is the shortness of service life of their electrodes, especially when the torches are used with an oxidizing arc gas, such as oxygen or air. In those torches, the gas appears to rapidly oxidize the copper, and as the copper oxidizes, its work function fails. As a result, the oxidized copper that surrounds the insert begins to support the arc in preference to the insert. After this occurs, the copper melts, thereby causing early destruction and/or failure of the electrode.
U.S. Pat. No. 5,023,425 (Severance, Jr.) which issued on Jun. 11, 1991, and which is incorporated herein by reference, discloses an electrode for a plasma arc torch wherein the electrode includes a copper holder having a lower end which mounts an emissive insert that acts as the cathode terminal for the arc during operation. A sleeve of silver is positioned to surround the insert and forms an annular ring on the lower end surface of the holder to surround the exposed end face of the emissive insert. The annular ring serves to prevent arcing from the copper holder, and maintains the arc on the insert. However, while the silver sleeve of the '425 patent was intended to prolong the life of the copper holder, in practice, this electrode suffers from problems in that the wear does not come from double arcing, but from the hafnium overheating and eroding.
U.S. Pat. No. 3,930,139 (Bykhovsky et al.) which issued on Dec. 30, 1975, and which is incorporated herein by reference, also discloses an electrode for plasma arc working of materials. In the '139 patent, the holder is again formed from copper or copper alloys and an active insert is fastened to the end face of the holder and is in thermal and electrical contact with the holder through a metal distance piece disposed between the active insert and the holder and over the entire contact surface area. The metal distance piece is formed from aluminum or aluminum alloys and the active insert is formed from hafnium or from hafnium with yttrium and neodymium oxides as dopants therein taken separately or in combination. However, while the aluminum sleeve surrounding the active insert in the '139 patent serves to protect the copper holder surrounding the active insert, the aluminum distance piece or sleeve offers no advantages over the silver sleeve of the '425 patent to Severance, Jr.
U.S. Pat. No. 5,676,864 (Walters) which issued on Oct. 14, 1997, which is incorporated herein by reference, discloses an electrode for a plasma arc torch wherein the electrode includes a copper holder having a lower end which mounts an emissive insert that acts as the cathode terminal for the arc during operation. A sleeve of silver is positioned substantially to surround the insert and form an annular ring on the lower end surface of the holder to surround the exposed end face of the emissive insert. The insert assembly further includes an aluminum face plate disposed in the enlarged outer portion of the cavity and which is exposed at the front end of the metallic holder so as to surround a front portion of the sleeve.
U.S. Pat. No. 5,767,478 (Walters) which issued on Jun. 16, 1998, alternatively teaches eliminating the aluminum face plate of U.S. Pat. No. 5,676,864 and instead provides for the front end of the holder to directly contact the emissive insert forming an overlay portion of the holder between the front face thereof and the sleeve, thus protecting the silver sleeve.